Neurodegenerative disorder has become a rapidly spreading health crisis in the United States. It affects millions of Americans today, and costs hundreds of billions of dollars to provide caring for the patients annually. Despite the tremendous effort in research aim to discover new medicines, most neurodegenerative diseases remain incurable. According to this situation, there is an urgent need for the development of novel neurodegenerative therapeutic agents. Historically, the abnormally high concentration of cerebral nitric oxide (NO), the product of the neuronal isoform of nitric oxide syntheses (nNOS), has been observed as a key pathogenic phenomenon shared by various neurodegenerative conditions including chronic pathologies such as Alzheimer's, Parkinson's, Huntington's, headaches, as well as neuronal damage in stroke. Such observations provide a rationale for the therapeutic use of nNOS inhibitors as a general treatment for neurodegeneration. In the past two decades, a large number of nNOS inhibitors have been reported, but most have met with limited success.
Recently, cis-3,4-pyrrolidine inhibitors (e.g., 1 and 2, FIG. 1) were developed using structure-based drug design. (See, e.g., Silverman, R. B. Design of selective neuronal nitric oxide synthase inhibitors for the prevention and treatment of neurodegenerative diseases. Acc. Chem. Res. 2009, 42, 439 451; Delker, D. L.; Ji, H.; Li, H.; Jamal, J.; Fang, J.; Xue, F.; Silverman, R. B.; Poulos, T. L. Unexpected binding modes of nitric oxide synthase inhibitors effective in the prevention of cerebral palsy. J. Am. Chem. Soc. 2010, 132, 5437 5442; Ji, H.; Tan, S.; Igarashi, J.; Li, H.; Derrick, M.; Martásek, P.; Roman, L. J.; Vásquez Vivar, J.; Poulos, T. L.; Silverman, R. B. Selective neuronal nitric oxide synthase inhibitors for prevention of cerebral palsy. Ann. Neural. 2009, 65, 209 217; and Lawton, G. R.; Ranaivo, H. R.; Wing, L. K.; Ji, H.; Xue, F.; Martesek, P.; Roman, L. J.; Watterson, D. M.; Silverman, R. B. Analogues of 2-aminopyridine-based selective inhibitors of neuronal nitric oxide synthase with increased bioavailability. Bioorg. Med. Chem. 2009, 17, 2371 2380.) These inhibitors showed excellent potency and isoform selectivity for nNOS over endothelial NOS and inducible NOS, which made them promising candidates for treatment of neurodegenerative diseases. To target nNOS and exhibit pharmaceutical activity in the brain, such candidates also need to penetrate the blood brain barrier (BBB), a unique barrier formed by brain capillary endothelial cells. However, animal test results indicated that the pyrrolidine inhibitors (e.g., 1 and 2) do not penetrate the BBB. Presumably, from a structural perspective, the amino groups of 1 and 2 are positively charged at physiological pH, inhibiting BBB penetration.
To circumvent this challenge, different strategies have been developed to tune the chemical structures of 1 and 2 and increase lipophilicity without impairing potency and selectivity. For example, the high pKa amino groups were replaced by neutral functionalities such as ethers and amides. (See, Lawton, G. R.; Ranaivo, H. R.; Wing, L. K.; Ji, H.; Xue, F.; Martesek, P.; Roman, L. J.; Watterson, D. M.; Silverman, R. B. Analogues of 2-aminopyridine-based selective inhibitors of neuronal nitric oxide synthase with increased bioavailability. Bioorg. Med. Chem. 2009, 17, 2371-2380.) Different electron-withdrawing groups, such as ether, difluoromethylene, and cyclopropyl groups, were incorporated next to the amino group in the lipophilic tail to partially remove the positive charge of the amino group at physiological pH. (See, Xue, F.; Fang, J.; Lewis, W. W.; Martasek, P.; Roman, L. J.; Silverman, R. B. Potent and selective neuronal nitric oxide synthase inhibitors with improved cellular permeability. Bioorg. Med. Chem. Lett. 2010, 20, 554 557; and Xue, F.; Huang, J.; Ji, H.; Fang, J.; Li, H.; Martásek, P.; Roman, L. J.; Poulos, T. P.; Silverman, R. B. Structure-based design, synthesis, and biological evaluation of lipophilic-tailed monocationic inhibitors of neuronal nitric oxide synthase. Bioorg. Med. Chem. 2010, 18, 6526-6537). While such modifications provided a new generation of pyrrolidine inhibitors with improved membrane and BBB permeability, the potency and isoform selectivity of some such components were somewhat compromised.